Human immunodeficiency virus (HIV) infections within the brain produce serious neurological complications. It is estimated that at least 30% of all adults with AIDS and 50% of all children with AIDS will suffer central nervous system disorders related to HIV infection within the brain. Treatment of the neurological complications related to AIDS is difficult as anti-viral dideoxynucleosides commonly used in the treatment of AIDS do not readily cross the blood-brain barrier (BBB). The presence of carrier systems for 3'-azido-2',3'-dideoxythymidine (AZT) and related dideoxynucleosides in the blood-cerebral spinal fluid barrier have been well documented. However, few studies have critically evaluated the presence of dideoxynucleoside carriers within the brain microvessel endothelial cells that form the BBB. In the present proposal, carrier systems mediating the transport of dideoxynucleosides both into and out of brain microvessel endothelial cells will be examines. The working hypothesis for the proposed studies are that specific, distinct carrier systems exist on the plasma membrane of the brain microvessel endothelial cells that control the uptake and efflux of dideoxynucleosides. Dideoxynucleoside carriers on the BBB will be identified and characterized using primary cultured bovine brain microvessel endothelial cells (BBMEC). To examine uptake carriers, the accumulation of a series of radiolabeled dideoxynucleosides (AZT, dideoxyinosine, dideoxycytosine, and dideoxythymidine) in BBMEC monolayers will be examined to determine: 1) the uptake rates for each compound, 2) the saturability of uptake and structural features required for nucleoside interactions with the carrier, 3) the energy-dependency of uptake and 4) the localization of carriers on the lumenal (blood side) and ablumenal (brain side) plasma membrane. Efflux of the same series of radiolabeled dideoxynucleosides out of BBMEC monolayers will be examined to determine: 1) the susceptibility of dideoxynucleoside efflux to known nucleoside carrier inhibitors, 2) important structural features required for dideoxynucleoside interactions with the efflux carrier, and 3) the location and distribution of dideoxynucleoside carriers on the lumenal and ablumenal side of BBMEC monolayers. These studies will critically examine the cellular mechanisms controlling the passage of dideoxynucleosides across the BBB. By characterizing uptake and efflux carriers with regard to their specificity of inhibitors and their energy requirements, comparisons can be made to nucleoside transport systems in other cells. The information obtained in these studies can also be used to improve the efficacy of both current and future anti-viral nucleosides in the treatment of neurological disorders associated with AIDS.